Re: [PATCH v1 1/2] scsi: ufs: Fix unbalanced scsi_block_reqs_cnt caused by ufshcd_hold()

From: Stanley Chu
Date: Tue Nov 03 2020 - 09:04:58 EST


Hi Can,

On Tue, 2020-11-03 at 18:01 +0800, Can Guo wrote:
> On 2020-11-03 15:07, Stanley Chu wrote:
> > Hi Can,
> >
> > On Mon, 2020-11-02 at 22:24 -0800, Can Guo wrote:
> >> The scsi_block_reqs_cnt increased in ufshcd_hold() is supposed to be
> >> decreased back in ufshcd_ungate_work() in a paired way. However, if
> >> specific ufshcd_hold/release sequences are met, it is possible that
> >> scsi_block_reqs_cnt is increased twice but only one ungate work is
> >> queued. To make sure scsi_block_reqs_cnt is handled by ufshcd_hold()
> >> and
> >
> > Just curious that how could this be possible? Would you have some
> > failed
> > examples?
> >
>
> [1] One gate_work() is in the workqueue, not yet executed, now clk state
> == REQ_CLKS_OFF.
> [2] ufshcd_queuecommand() calls ufshcd_hold(async == ture) ->
> active_req++ -> scsi_block_reqs_cnt++ -> REQ_CLKS_ON -> queue ungate
> work -> active_req-- -> return -EAGAIN.
> [3] Now gate_work() starts to run, but since the clk state is
> REQ_CLKS_ON, gate_work() just sets clk state to CLKS_ON and bail.
> [3] Someone calls ufshcd_hold(async == false) -> do something ->
> ufshcd_release() -> clk state is changed to REQ_CLKS_OFF. Note that,
> till now, ungate_work() is still in the work queue, not executed yet.
> [4] Now, if someone calls ufshcd_hold(), we will hit the issue.
>
> Above sequence is a very common clk gate/ungate sequence. The issue
> is because ungate_work is queued but cannot be executed in time. In my
> case, I see the ungate_work is somehow delayed for about 150ms. This
> change has been tested by customers on multiple platforms. And you
> can tell from the code that it won't break anything. :)

Thanks so much for the details. Looks good to me.

Reviewed-by: Stanley Chu <stanley.chu@xxxxxxxxxxxx>

>
> Thanks,
>
> Can Guo.
>
> >> ufshcd_ungate_work() in a paired way, increase it only if queue_work()
> >> returns true.
> >>
> >> Signed-off-by: Can Guo <cang@xxxxxxxxxxxxxx>
> >> Reviewed-by: Hongwu Su <hongwus@xxxxxxxxxxxxxx>
> >> ---
> >> drivers/scsi/ufs/ufshcd.c | 6 +++---
> >> 1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> >> index 847f355..efa7d86 100644
> >> --- a/drivers/scsi/ufs/ufshcd.c
> >> +++ b/drivers/scsi/ufs/ufshcd.c
> >> @@ -1634,12 +1634,12 @@ int ufshcd_hold(struct ufs_hba *hba, bool
> >> async)
> >> */
> >> /* fallthrough */
> >> case CLKS_OFF:
> >> - ufshcd_scsi_block_requests(hba);
> >> hba->clk_gating.state = REQ_CLKS_ON;
> >> trace_ufshcd_clk_gating(dev_name(hba->dev),
> >> hba->clk_gating.state);
> >> - queue_work(hba->clk_gating.clk_gating_workq,
> >> - &hba->clk_gating.ungate_work);
> >> + if (queue_work(hba->clk_gating.clk_gating_workq,
> >> + &hba->clk_gating.ungate_work))
> >> + ufshcd_scsi_block_requests(hba);
> >> /*
> >> * fall through to check if we should wait for this
> >> * work to be done or not.
> >
> > Thanks,
> > Stanley Chu